1. Field of the Invention
The present invention relates to a multi-ply paper as well as to a method and an apparatus for the manufacture of such multi-ply paper.
2. Description of the Related Art
In the manufacture of paper with specific application profiles, notably packing papers, attempts are presently being made at producing papers which in their Z-axis (perpendicular to the surface of the vertical axis) depend heavily on depth as regards their fiber and solids concentration. For ones attempts are directed at embedding, with the aid of multilayer headboxes, low-cost materials (wastepaper basis) in the middle ply of the paper and applying the high-quality, or expensive materials only on the outer plies or one outer ply where they substantially determine the quality of the paper, e.g., to make them suited for printing.
Reference is made to European Patent Document No. EP 0 651 092 A1. This document teaches a multilayer headbox and a method for the manufacture of a multi-ply paper with which it is possible to specifically enrich the stock suspension of the outer layers with the desired materials. Thereby, the desired profile of the concentration across the Z-axis is maximally approached, so as to obtain a maximally white surface of good printability on the paper top side. The desired profile has a low concentration in the center and a high concentration at the edges, also known as a "smiley profile."
In another method, several headboxes (secondary headboxes) successively apply individual paper layers with different properties on one or several fourdrinier wires, thus creating a multi-ply paper. The multi-ply paper includes a high-quality, mostly bleached and printable chemical wood pulp layer (e.g., "white top liner") on at least one side.
Problems associated with all of the above methods are that very much dewatering energy is necessary in forming the plies and that the machinery expense for the manufacture is considerable. The biggest problem, however, is that all of the above methods are based on the use of expensive bleached chemical wood pulp or deinked fiber stock as raw material.
A further problem is that there are many paper machines in operation which no longer satisfy today's needs with respect to the quality of the papers they produce.